The slew rate (SR) is a maximum ratio of output voltage of an amplifier to time, i.e., the maximum rate of change of an operational amplifier. A low slew rate is one of important factors limiting high-speed applications of the operational amplifier. Conventional folded cascode operational amplifiers usually increase the bias current value of the current source in integrated circuit (IC) to gain a high slew rate.
FIG. 1 shows a conventional folded cascode operational amplifier circuit. The current of tail current source M3 is I. When the input voltage of the circuit satisfies Vin+−Vin−√{square root over (2)}·Vdast—M4, the circuit works in a large signal state, an input-transistor M4 is conductive, a transistor M9 is turned off, and transistors M7 and M8 are turned off so that a transistor M5 is also turned off. Thus, the currents passing through M1, M3 and M4 are I. Because the width-to-length ratio and other parameters of a current source M2 are often similar to M1, the current passing through M2 is also I and it charges a load capacitor of output-terminal by transistor M6. The positive slew rate SRP=I/CL, where CL is the capacitance of the load capacitor.
On the other hand, when the input voltage of the circuit satisfies Vin−−Vin+>√{square root over (2)}·Vdast—M4, the load capacitor discharges, and the negative slew rate SRN=I/CL. Such conventional folded cascode operational amplifier usually increases the bias current value of the tail current source to gain a high slew rate.
However, when gaining the high slew rate by increasing the bias current, the power consumption may also increase by multiple folds. Therefore, such method may be unable to satisfy the requirement of low power consumption in high-speed IC applications. The disclosed methods and systems are directed to solve one or more problems set forth above and other problems.